The present invention relates to a cooling module and an apparatus for thermally treating substrates, in particular, solar cells having an ink or paste printed thereon.
In the solar cell technology it is known to print electrically conducting screen printing paste onto solar cell substrates for forming electrical conductors. Such paste or ink is typically initially dried after application thereof in a dryer module, in order to evaporate solvents. Subsequently, the substrates are thermally treated, in order to form the electrical characteristics of the paste to obtain electrical conductors. The substrates are typically heated to temperatures of approx. 900° C. Thereafter, the substrates are again cooled in a cooling module.
For a high throughput, the modules are typically formed as continuous modules and are arranged directly adjacent to each other. Typically, a single transport unit, for example in the shape of a transport belt, is provided, which transports these substrates consequently through the dryer module, the heating module and the cooling module. It is also possible, to separate the processes of drying, heating and/or cooling from the other of these processes and to provide a separate transport unit for each module.
Providing separate transport units, however, may lead to transfer problems between the transport units, and furthermore, separate transport belts would also require separate drives, which may increase the complexity of the overall apparatus.
Therefore, a single transport unit, which has a transport belt running through all modules is preferred. This, however, leads to the problem that within the heating module, not only the substrates to be treated but also the transport belt itself is heated, thereby increasing the cooling effort required within the cooling module. It is known to provide a water cooled stainless steel wall within the cooling module, which extends substantially parallel to the transport belt, in order to receive radiant heat. Furthermore, it is known to conduct pressurized air onto the substrates in order to additionally cool the same. The currently used stainless steel wall, however, does not have sufficient absorption characteristics in order to absorb the whole radiant heat, such that at least a part of the radiant heat of the substrates and/or of the transport belt is reflected back to the same.
Thus, relatively long cooling trucks are required until the substrates have a sufficiently reduced temperature, in order to be transferred to a downstream transport mechanism. Furthermore, the transport belt still has an increased temperature, which requires the use of special guide and drive elements. In particular, the transport belt typically has a temperature, which excludes the use of drive and/or guide rollers made of rubber or having a rubberized surface. Such rollers, however, would be advantageous for a smoother driving of the belt and with respect to the overall cost of the transport mechanism.